Process of purifying alpha, beta-unsaturated aliphatic aldehydes resulting from aldol condensation



rates Parent @fiiee fdfildfifi Patented Dec. 12, 1961 PROCES 0F PURHFYENG ALPHA,BETA-UNATU- RATED ALTEFATHI ALDEHYDES RESULTING FRGM ALDGL QGNDENSATIQN Hugh 3. Hagemeyer, 51a, Glenn V. Hudson, and Edward J. Mooney, Longview, Tex, assignors to Eastman Kodak (Iompany, Rochester, N.Y., a corporation of New Jersey No Drawing. Fiied May 27, 1953, Ser. No. 733,049

2 Claims. (Cl. 260-661) This invention relates to an improvement in the process of producing a1pha,beta-unsaturated aliphatic aldehytles by the aldol condensation of aliphatic aldehydes containing fewer carbon atoms. Specifically it relates to an improvement in the process of producing 4- to 12- carbon alpha,beta-unsaturated aliphatic aldehydes such as crotonaldehyde, 2,4-dimethylbuten-2-al, Z-ethylhexen-Z- al, Z-methylpenten-Z-al, 2-ethylisoheXen-2-al (2-ethyl-4- methylpenten-Z-al), 2-propylhepten-2-al, 2-propyl-4-methyIheXen-Z-al, 2-isopropyl-5-'nethylhexen-2-al, 2,4-dimethylpenten-2-al, 2-ethyl-3-methylheXen-2-al and 4-butylocten-Z-al by the aldol condensation of aliphatic aldehydes of from 2 to 6 carbon atoms in the presence of an alkali metal hydroxide catalyst. Such aldol condensations are described in copending application Serial No. 541,052 of Hagemeyer and Hudson, filed October 17, 1955, now U.S. Patent Number 2,852,563.

The 4- to 12-carbon alpha,beta-unsaturated aliphatic aldehydes can be hydrogenated to the corresponding saturated alcohols of the same number of carbon atoms. Some of the dicarboxylic acid esters of these alcohols are useful as plasticizers. For use in light-colored plastics it is necessary that the esters be free from color. Unless the alcohol is free from color, and remains free from color during the esterification process, the ester is discolored.

In the aldol condensation of aliphatic aldehydes to form alpha,beta-unsaturated aliphatic aldehydes of a greater number of carbon atoms, the aliphatic aldehydes are condensed with themselves or each other in the presence of 0.1% to by weight of an alkaline catalyst, in an aldol reactor which overflows to a decanter. The bottom layer (principally aqueous caustic) is recycled to the reactor. The top layer from the decanter contains the alpha,beta-unsaturated aliphatic aldehydes (enals) resulting from aldol condensation of the aldehydes and de hydration of the aldol formed, both of which reactions take place in the aldol reactor. These enals contain not only unreacted starting aldehydes, but high boiling components resulting from the aldol condensation, which steam distill in the same temperature range as do the enals and remain unchanged in the subsequent hydrogenation step. The exact nature of these high boiling components boiling or steam distilling with the enal has not in all cases been determined, but their presence in the alcohol refining step causes severe deterioration of the quality of the final product if they are not completely removed by distillation or chemical treatment.

We have discovered that the high-boiling by-products resulting from the aldol condensation of aliphatic aldehydes of 2 to 6 carbon atoms, which steam distill in the same temperature range as the enals, can be largely decomposed to the desired enal and/ or the original aliphatic aldehydes by the action of dilute caustic and heat. The crude product from the aldol reactor, comprising enals, unreacted starting aldehydes, and high-boiling components, is continuously fed to a steam distillation column and steam distilled continuously from an aqueous solution of an alkali, at base temperatures of 90-100 C. We have found that this treatment decomposes the highboiling by-products, largely into the desired enal and the starting aldehyde. After subsequent stripping out of the unreacted starting aldehydes, which can be recycled to the aldol reactor, the enal is sufiiciently pure so that hydrogenation of it, without further purification gives a high yield of the corresponding alcohol, of good quality. By removal of the high-boiling by-products at this early stage in the process, the capacity of the subsequent refining system is greatly increased, due to the increased throughput which can be processed Without sacrifice of quality of the final product.

In the production of Z-ethylhexanol by hydrogenation of the product of the aldol condensation of n-butyraldehyde, we have found among the by-products Z-ethylhexane-l,3-diol, the monobutyrate of 2-ethylheXane-1,3-diol, n-butyl-2-ethyl-3-hydroxyhexanoate, substituted valerolactones, and miscellaneous higher hydrogenated aldol products. By steam distilling the crude unsaturated aldehyde from a dilute caustic solution, formation of most of these products and particularly of the higher aldol condensation products is eliminated. Steam distillation of the crude aldol product from an alkaline medium sub stantially completes the cleavage of the aldehyde-aldol trimer to the aldol and the aldehyde monomer, the aldol being subsequently dehydrated to the enal and the aldehyde removed by a stripping process and recycled to the aldol reactor. This step substantially decreases the quantity of high and low boiler streams obtained in the final purification system for the C -C alcohols, as it avoids decomposition of the trimer under hydrogenating conditions in the autoclave to give low boiling alcohols and substituted diols. Further, by decreasing the quantity of aldol reaching the autoclave, it virtually eliminates one of the possible precursors of low boiling carbonyl compounds.

The feed to the steam distillation column contains the desired 4- to 12-carbon enal, unreacted aldehydes, aldehyde trimer, aldol and high-boiling condensation products. the concentration of caustic in the base of the steam distillation column may be anywhere Within the range of 0.1% to 10%, any concentration within this range giving marked improvement in the quality of the final product. We have found that caustic concentrations of from 1% to 3% produce near optimum results. The caustic may be sodium hydroxide or potassium hydroxide. The aqueous caustic solution is fed near the top of the steam distillation column and circulated through the column to obtain maximum contact of the organic layer with the caustic. The spent caustic is overflowed from the base of the steam distillation column to the ditch. Makeup caustic is added to the feed as needed. The concentration of salts resulting from oxidation of the aldehydes is maintained at from O10% of the aqueous layer. There is no build-up of organic material in the base of the steam distillation column, and no soluble organic materials other than salts of acids are discharged with the aqueous layer. When simple steam distillation is employed at this stage, a considerable quantity of insoluble non-volatile organic material builds up in the base heater and requires periodic draw-cits to maintain optimum operation.

The steam distillate is fed to a stripping column, where unreacted aldehydes are stripped out and recycled to the aldol reactor. The overflow from the base of the stripping column is fed to a continuous autoclave, Where it is hydrogenated in the presence of a Raney nickel catalyst. The discharge from the autoclave is filtered and washed, then charged to a distillation column Whose function is to dry the alcohol and remove any low boilers present.

Although the object of the hydrogenation is to reduce the unsaturated aldehyde (enal) to the corresponding saturated alcohol (anol), some of the enal is reduced only to the corresponding saturated aldehyde (anal). Therefore, the overflow from the base of the drying column is charged to an anal recovery column, where the saturated aldehyde formed is recovered and recycled to the autoclave for further reduction.

The overflow from the base of the anal recovery column is fed to the primary refining column. The. top takeoil from this column is the refined saturated alcohol; If the alcohol is synthesized by the reaction of two molecules of the same aldehyde, as in the case of Z-ethylhexanol, the refining system ends here. However, if the product alcohol results from the condensation of two dissimilar aldehydes, as in the case of 2-ethyl-isohexanol (from n-butyraldehyde plus isobutyraldehyde), the overflow from the primary refining column is fed to a secondary refining column. The top take-off from this column is returned to the primary refining column, while the overflow from the base is fed to a column for the recovery of the other alcohol or alcohols formed coincident with the synthesis of the desired alcohol. For instance,- where Z-ethyl-isohexanol is the desired product, its formation is accompanied by the formation of a certain quantity of Z-ethyl-hexanol, which would be the product recovered in the last column. The overflow from the base of the final column consists of high boiling compounds.

The invention is further illustrated by the following specific examples.

Example 1.-In the manufacture of 2-ethylhexanol from n-butyraldehyde by the process outlined above, when the crude enal from the aldol reactor was simply steam distilled, an average weekly material balance over a production unit indicated that 28,231 pounds of Z-ethylhexanol were produced daily. This was the maximum capacity of the unit consistent with production of fair quality product. By-products of this production were 1,650 pounds of high boilers and 432 pounds of low boilers: a total of 2,082 pounds of nonsalable product. As a measure of the quality of this product, color stability of the alcohol under esteriflcation conditions was tested by dioctyl phthalate (DOP) color test and sulfuric acid reflux color test. The sulfuric acid reflux color of the alcohol was 60-85 ppm. on the APHA scale; the APHA color of the di-Z-ethylhexyl phthalate (DOP) prepared from the 2-ethylhexanol was 75-100 ppm.

When the crude enal from the same aldol reactor in the same production unit was steam distilled from aqueous sodium hydroxide of from 1-3% conentration, the sodium hydroxide solution being circulated through the steam distillation column, as described above, the same production unit produced daily 45,839 pounds of 2-ethylhexanol with a sulfuric acid reflux color of 15-30 p.p.m. and a DO? color of -35 ppm. 383 pounds of high boilers and 521 pounds of low boilers were produced daily: a total of 904 pounds of nonsalable product.

The initials APi-IA stand for American Public Health Association. The measurement of APHA color, on a Pt-Co scale, is described on page 673 of the book Technical Methods of Analysis, by Roger C. Griffin, published in 1927 by McGraw-HilL The sulfuric acid refiux color test and the DO? color test are performed as follows.

Sulfuric acid reflux color test-Place 100 ml. of the sample in a 300-ml. round bottom flask with standard taper, ground glass joint. While vigorously swirling the sample, slowly pipette 2 ml. of concentrated sulfuric acid into the flask and add 2 or 3 carborundurn chips. Attach a water cooled reflux condenser of at least 200 mm. length, immediately affix an electric heating mantle to the flask, and bring the contents to a boil. Reflux for 50 minutes, measuring time from the instant the first drop falls from the condenser. Remove the flask and cool as rapidly as possible to about 70 C. by swirling first under warm tap water and then under cold tap water. Immediately fill a Nessler tube to the mark with the resulting test mixture and compare the color with the standard tubes of :the platinum-cobalt scale, as described in the Grifiin reference, to the nearest 5 ppm. The sulfuric acid is reagent grade.

DOP color ten-Weigh 148 g. of phthalic anhydride into a 1000 ml. round bottom flask equipped with a ground glass joint. Add 364 g. of the sample to the flask containing the phthalic anhydride. While swirling the flask, add 0.1 ml. of concentrated H 30 from a 1 ml. pipette graduated in 0.1 m1. Add 2 or 3 carborundum chips to the flask.- Attach a Dean-Stark moisture receiver, equipped with a stopcock for draining water, to the flask. Fill the receiver with additional sample until a drop overflows into the flask. Attach a reflux condenser at least 200 min. in length to the Dean-Stark receiver. Aflix a hemispherical heating mantle to the flask and heat rapidly, with intermittent shaking and swirling, until the phthalic anhydride melts. Reflux the mixture for exactly 30 minutes, measuring time from theinstant the first drop falls from the condenser tip. Do not allow more than 5 ml. of water to accumulate in the Dean- Stark trap. Remove the flask and pour more than 50 m1. of the hot solution into a 250 ml. Erlenmeyer flask. Cool as rapidly as possible to 70 C. by swirling it first under war tap water and then under cool tap water. Immediately fill a Nessler tube to the marl; with the resulting test mixture and compare the color with the standard tubes of the platinum-cobalt scale, as described in the Griflin reference, to the nearest 5 ppm. The phthalic anhydride and sulfuric acid are reagent grade. This test is customarily referred to as the BOP test, no matter what the alcohol to which it is applied.

Example 2.ln the manufacture of Z-ethylisohexanol from n-butyraldehyde and isobutyraldehyde by the process outlined above, when the crude enal from the aldol reactor was simply steam distilled, a production unit produced an average of 16,720 pounds of Z-ethylisohexanol daily. This was the approximate maximum production possible in this equipment consistent with generally good quality. By-products of this production were 1,593 pounds of high boilers and 432 pounds of low boilers: a total of 2024 pounds of nonsalable product. The acid reflux color test of the alcohol was -110 ppm. on the APHA scale; the APHA color of the di-2-ethylisohexyl phthalate (DIOP) made in the BOP test was -150 ppm.

When the crude enal from the same aldol reactor in the same production unit was steam distilled from aqueous sodium hydroxide of from 1-3% concentration, the sodium hydroxide solution being circulated through the steam distillation column, as described above, the same production unit produced 25,401 pounds per day of Z-ethylisohexanol with an acid reflux color of 30-60 ppm. and a DOP color of 30-55 ppm. 750 pounds of high boilers and 320 pounds of low boilers were produced daily: a totalof 1070 pounds of nonsalable product.

Example 3.-A laboratory batch preparation of 2- propylinethylhexen-Z-al (from n-valeraldehyde and 2- rnethyl-n'butyraldehyde) was divided into two portions. One portion was steam distilled from pure water, and the second portion was steam distilled from a 2% aqueous solution of sodium hydroxide. Some organic residue remained in the distillation from pure water. The steam distillate from each sample was hydrogenated, and 1500 grams of the hydrogenated product from each experiment was refined in identical fashion. From the portion which had been steam distilled from pure water we obtained 1270 grams of C alcohol, 15 8 grams of high boilers, and 68 grams of low boilers. APHA color of the phthalate ester of the alcohol was ppm. From the portion which had been steam distilled from caustic solution, we obtained 1335 grams of C alcohol, 118 grams of high boilers, and 42 grams of low boilers. APE-1A color of the phtlralate ester of the alcohol was 80.

Example 4.-A pilot plant unit for making 2-propylheptanol through the aldol condensation of n-valderaldehyde produced daily 1150 pounds of C alcohol, 95

pounds of high boilers and 68 pounds or" low boilers when the crude enal was being steam distilled from pure Water. The phthalate ester of the alcohol had an APHA color of 110 ppm. Steam distillation of the crude coal from aqueous sodium hydroxide of from 1% to 3% concentration increased daily production to 1300 pounds of C alcohol, with 63 pounds of high boilers and 47 pounds of low boilers. The phthalate ester had a color of 25 ppm. on the APHA scale.

Example 5.-A pilot plant unit for making n-butanol through the aldol condensation of acetaldehyde produced daily 960 pounds of C alcohol, 153 pounds of high boilers and 70 pounds of low boilers When the crude enal was being steam distilled from pure water. The phthalate ester of the alcohol had an APHA color of 95 ppm. Steam distillation of the crude enal from aqueous sodium hydroxide of from 1% to 3% concentration increased daily production to 1240 pounds of C alcohol, with 75 pounds of high boilers and 52 pounds of low boilers. The phthalate ester of the alcohol had a color of 30 p.p.m. on the APHA scale.

Example 6.-A laboratory batch preparation of 2-buty1- octen-Z-al (from n-hexaldehyde) was divided'into two portions. One portion was steam distilled from pure water, and the second portion was steam distilled from a 2% aqueous solution of sodium hydroxide. The steam distillate from each sample was hydrogenated, and 1100 grams of the hydrogenated product from each experiment was refined in an identical manner. From the portion which had been steam distilled from pure water we obtained 883 grams of C alcohol, 139 grams of high boilers, and 78 grams of low boilers. The phthalate ester of the alcohol had an APHA color of 175 p.p.m. From the portion which had been steam distilled from caustic solution we obtained 975 grams of C alcohol, 73 grams of high boilers and 52 grams of low boilers. The phthalate ester of the alcohol had an APHA color of p.p.m.

What we claim as our invention and desire to be 86- cured by Letters Patent of the United States is:

1. In a process or" manufacturing alpha,beta-unsaturated aliphatic aldehydes of from 4 to 12 carbon atoms by aldol condensation of aliphatic aldehydes of from 2 to 6 carbon atoms and dehydration of the resulting aldols, the step of purifying the resulting alpha,beta-unsaturated aldehyde by steam distilling it from an aqueous solution 1 of caustic alkali of from 0.1% to 10% concentration.

2. In a process of manufacturing alpha,beta-unsatu rated aliphatic aldehydes of from 4 to 12 carbon atoms by aldol condensation of aliphatic aldehydes of from 2 to 6 carbon atoms and dehydration of theresulting aldols, the step of purifying the resulting alpha,beta-unsaturated aldehyde by steam distilling it from an aqueous solution of caustic alkali of from 1% to 3% concentration.

References Cited in the file of this patent UNITED STATES PATENTS 1,738,659 Maude Dec. 10, 1929 2,096,637 Guinot Oct. 19, 1937 2,852,563 Hagemeyer et al Sept. 16, 1958 OTHER REFERENCES Weissberger: Technique of Organic Chemistry, volume IV, Distillation, pp. 374-378, Interscience Publishers, Inc., New York, 1951. 

1. IN A PROCESS OF MANUFACTURING ALPHA,BETA-UNSATURATED ALIPHATIC ALDEHYDES OF FROM 4 TO 12 CARBON ATOMS BY ALDOL CONDENSATION OF ALIPHATIC ALDEHYDES OF FROM 2 TO 6 CARBON ATOMS AND DEHYDRATION OF THE RESULTING ALDOLS, THE STEP OF PURIFYING THE RESULTING ALPHA,BETA-UNSATURATED ALDEHYDE BY STEAN DISTILLING IT FRON AN AQUEOUS SOLUTION OF CAUSTIC ALKALI OF FROM 0.1% TO 10% CONCENTRATION. 